Construction machine, typically, a hydraulic shovel includes a base carrier formed of a center frame which serves as a base, crawler frames, crawler tracks provided on the crawler frames, and a drive mechanism therefor. In general, the center frame and the crawler frames are assembled by bolting leg portions protruding from both sides of the center frame to an upper surface of the crawler frames. Such a structure allows the machinery to be dismantled for transportation.
FIG. 6 shows a specific example of a joint portion between leg portions 51 of a center frame 50 and a crawler frame 55. In this exemplary connection, mounting seat portions 52 protrude from both sides of the leg portions 51 having box constructions and mounting seats 56 provided on an upper surface of the crawler frame 55. The mounting seat portions 52 and mounting seats 56 overlap each other and are fastened by a number of mounting bolts. To tips of the leg portions 51 of the center frame 50, plates 53 are welded so as to close an opening of the box constructions in order to prevent sand from entering the leg portions 51 while the machinery is running on the site. A portion connected in this way can be easily damaged due to an influence of an external force which may be applied by a movement of the device while it is working. Thus, in order to reinforce the joint portions, Japanese Laid-Open Publication No. 9-136662proposes a joint structure in which members with a number of mounting bolt holes are provided on a series of block members for bolt fastening.
However, in the structure shown in FIG. 6, when the machinery goes over bumps while it is working or running on a sloping land, a force in a helical direction is applied to the crawler frame. A large external force is applied to the tip portions of the leg portions in welded portions between upper members 51a, vertical members 51b and lower members 51c of the leg portions 51 which are formed into a box construction, and damage may be caused. Since the plates 53 attached to the tips of the leg portions 51 are strength members, a high stress is applied to the welded portions and the base material of the plate 53. This causes a problem that, due to an influence from the plates 53 attached to the tips, a crack is easily generated around side portions.
The reinforcing means known from Japanese Laid-Open Publication No. 9-136662 has been proposed in order to replace conventional reinforcing ribs (gussets) which connect vertical members forming protruding leg portions of the center frame and lower members which serve as mounting seats because such conventional ribs can easily be damaged due to concentration of a stress. This problem is solved by the proposed means. However, the method may cause another problem. In this method, tie bolts between the leg portions and a track frame (crawler frame) are bolted through mounting holes provided on one long block member used along the joint portion between the vertical plates of the leg portions and protruding portions of the leg portions (mounting plate). Thus, without sufficient control on fastening strengths of bolts, even when there is insufficiency in a fastening force for some of a plurality of bolts, this may be overlooked, and damage of a tie bolt or a crack in a welded portion near a protruding portion of a leg portion may occur.